Many combustion processes generate effluent gases having an unacceptable NOX content. Thus, oxides of nitrogen are one of the principal contaminants emitted by combustion processes. These compounds are found in stack gases mainly as nitric oxide (NO) with lesser amounts of nitrogen dioxide (NO.sub.2) and only traces of other oxides. Since nitric oxide (NO) continues to oxidize to nitrogen dioxide (NO.sub.2) in the air at ordinary temperatures, there is no way to predict with accuracy the amounts of each separately in vented gases at a given time. Thus, the total amount of nitric oxide (NO) plus nitrogen dioxide (NO.sub.2) in a sample is determined and referred to as "oxides of nitrogen (NOX)".
Oxides of nitrogen emissions from stack gases, through atmospheric reactions, produce "smog" that stings eyes and causes acid rains. For these reasons, the content of oxides of nitrogen present in gases vented to the atmosphere is severely limited by various state and federal agencies. To meet the regulations for NOX emissions, several methods of NOX control have been employed. These can be classified as either equipment modification or injection methods. Injection methods include injection of either water or steam to lower the temperature since the amount of NOX formed generally increases with increasing temperatures, or injection of ammonia to selectively reduce NOX. Water or steam injection, however, adversely affects the overall fuel efficiency of the process.
A process involving the injection of ammonia into the products of combustion is shown, for example, in Welty, U.S. Pat. No. 4,164,546. Examples of processes utilizing ammonia injection and a reducing catalyst are disclosed in Sakari et al, U.S. Pat. No. 4,106,286; and Haeflich, U.S. Pat. No. 4,572,110. Selective reduction methods using ammonia injection are expensive and somewhat difficult to control. Thus, these methods have the inherent problem of requiring that the ammonia injection be carefully controlled so as not to inject too much and create a possible emission problem by emitting excess levels of ammonia. In addition the temperature necessary for the reduction of the oxides of nitrogen must be carefully controlled to get the required reaction rates.
Equipment modifications include modifications to the burner or firebox to reduce the formation of NOX. Although these methods do reduce the level of NOX, each has its own drawbacks. A selective catalytic reduction system is presently considered by some authorities to be the best available control technology for the reduction of NOX. Currently available selective catalytic reduction systems used for the reduction of NOX employ ammonia injection into the exhaust gas stream for reaction with the NOX in the presence of a catalyst to produce nitrogen and water vapor. Such systems typically have an efficiency of 80-90 percent when the gas stream is at temperature within a temperature range of approximately 600.degree.-700.degree. F. The NOX reduction efficiency of the system will be significantly less if the temperature is outside the stated temperature range and the catalyst may be damaged at higher temperatures. As the present inventor R. D. Bell has disclosed in McGill et al U.S. Pat. No. 4,405,587, of which he is a co-patentee, oxides of nitrogen can be reduced by reaction in a reducing atmosphere such as disclosed in that patent at temperatures in excess of 2000.degree. F.
An important source of NOX emissions is found in the field of steam generation in direct-fired boilers. Excessive NOX emissions from such combustion are a serious environmental problem and various efforts to suppress them, such as the techniques referred to above, have been attempted, with varying results.
It is, accordingly, an object of this invention to provide an improved method involving combustion which brings about effective lowering of NOX in the combustion emissions and subsequent treatment to produce an acceptable final emission.
It is another object of the invention to provide a system for combustion in fired steam-generating boilers wherein final emissions will have significantly lowered NOX levels and be environmentally acceptable.